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OSPF
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Using the area ID to assign the OSPF area number
The OSPF area number is defined in the
areaid
<IP address>
option. The octet format is used in
order to be compatible with two different systems of notation used by other OSPF network vendors. There
are two valid ways to designate an area ID:
•
Placing the area number in the last octet (0.0.0.
n
)
Most common OSPF vendors express the area ID number as a single number. For example, the
Cisco IOS-based router command
Cisco IOS-based router command
network 1.1.1.0 0.0.0.255 area 1
defines the area
number simply as
area 1
. On the switch, using the last octet in the area ID,
area 1
is equivalent
to
areaid 0.0.0.1
.
•
Multi-octet (
IP address
)
Some OSPF vendors express the area ID number in multi-octet format. For example,
area 2.2.2.2
represents OSPF area 2 and can be specified directly on the switch as
areaid 2.2.2.2
.
NOTE:
Although both types of area ID formats are supported, be sure that the area IDs are in the
same format throughout an area.
Attaching an area to a network
Once an OSPF area has been defined, it must be associated with a network. To attach the area to a
network, you must assign the OSPF area index to an IP interface that participates in the area. The format
for the command is as follows:
>> # /cfg/l3/ospf/if <interface number>/aindex <area index>
For example, the following commands could be used to configure IP interface 14 for a presence on the
10.10.10.1/24 network, to define OSPF area 1, and to attach the area to the network:
>> # /cfg/l3/if 14 (Select menu for IP interface 14)
>> IP Interface 14# addr 10.10.10.1(Define IP address on backbone
network)
>> IP Interface 14# mask 255.255.255.0(Define IP mask on backbone)
>> IP Interface 14# ena (Enable IP interface 14)
>> IP Interface 14# ../ospf/aindex 1(Select menu for area index 1)
>> OSPF Area (index) 1 # areaid 0.0.0.1(Define area ID as OSPF area 1)
>> OSPF Area (index) 1 # ena (Enable area index 1)
>> OSPF Area (index) 1 # ../if 14 (Select OSPF menu for interface 14)
>> OSPF Interface 14# aindex 1 (Attach area to network on
interface 14)
>> OSPF Interface 14# enable (Enable interface 14 for area index 1)
Interface cost
The OSPF link-state algorithm (Dijkstra’s algorithm) places each routing device at the root of a tree and
determines the cumulative cost required to reach each destination. Usually, the cost is inversely
proportional to the bandwidth of the interface. Low cost indicates high bandwidth. You can manually
enter the cost for the output route with the following command:
enter the cost for the output route with the following command:
>> # /cfg/l3/ospf/if <OSPF interface number>/cost <cost value (1-65535)>